Today's cellular communication occurs mainly in frequency bands below 3 GHz. However, while LTE can operate over bandwidths of as much as 100 MHz by design, the future radio access system envisaged would operate over bandwidths of the order of 1 GHz. Clearly, such a system could not operate in bands below 3 GHz. The lowest band where the mobile industry may home for spectrum parcels that exceed the 10-40 MHz of contiguous allocations typical for the industry is probably above 3 GHz. Out of the regions of spectrum that are most promising for the mobile industry, the cm-Wave, CMW, region from 3-30 GHz and the mm-Wave, MMW, region from 30-300 GHz are particularly interesting for next generation mobile systems.
Furthermore, the IEEE 802.11 standardization effort is planning amendments termed IEEE 802.11ac and IEEE 802.11ad that will enable very high throughput communication over bandwidths such as 160 MHz for the former and 2 GHz for the latter. 802.11ac will operate in the CMW bands such as the 5 GHz ISM band while 802.11ad is targeting the 60 GHz unlicensed band.
Without specifying the exact band where we would operate the future radio access system, the next standard is assumed to operate over bandwidths that range from 100 MHz to 2.5 GHz in dense deployments.
The resulting system can be used in a variety of scenarios:                1) Point-to-point communications for short range radio systems        2) Access links for a Future Radio Access, FRA, system that provides very high speed connectivity or        3) Backhaul links between densely deployed nodes that provide a high throughput pipeline to a network operator's core network; this core network would connect to the Internet and provide access to data and multimedia services.        
One of the challenges of operating at MMW frequencies is the received power that decreases with frequency when using omnidirectional antennas because the antenna aperture—which determines how much power is received—decreases with frequency for an omnidirectional antenna and thus also the received power. To overcome this problem antenna area can be increased leading to directive antennas. Generally speaking, directive antennas and beam forming become an important component for MMW communication.
CSMA/CD
Carrier Sense Multiple Access/Collision Avoidance, CSMA/CA, is a contention based medium access mechanism used in the 802.11 standards to allow distributed coordination of the resources among users contending for the medium. In this disclosure CSMA/CD is referred to as an example of a contention based MAC protocol. CSMA/CD is therefore briefly described.
FIG. 1 illustrates a four-way handshaking in a CSMA/CA system based on Request-To-Send/Clear-To-Send, RTS/CTS. In FIG. 1, a first node, user A, wants to send a data packet to another node, user B. User A then sends an request to send, RTS, to the intended receiver. If the receiver is ready to receive, it broadcasts a clear to send, CTS, message. After receiving the CTS, the sender transmits the packet. All other nodes that receive the RTS or CTS refrain from transmission. This mechanism addresses the hidden/exposed terminal problem, described below.
To control the access to the medium, CSMA/CA uses inter-frame spaces, IFS, during which a node has to wait before sensing the channel and determining whether it is free. The 802.11 standard specifies different IFSs to represent different priority levels for the channel access: the shorter the IFS, the higher the priority. For instance, Short IFS, SIFS, is used for immediate acknowledgement of a data frame and Distributed Coordination Function IFS, DIFS, is used to gain access to the medium to transmit data, as further illustrated in FIG. 1.
Furthermore, to allow virtual carrier sensing, every data frame may contain the time needed for its transmission including the ACK, based on this information other nodes, here user C, will maintain a Network Allocation Vector, shown as NAV in FIG. 1, to determine when they should sense the medium again. The NAV is decremented by clock and no access is allowed as long as its value is above 0. The node will again sense the medium after NAV and the subsequent DIFS.
In addition, in order to avoid situations where two nodes transmit at the same time leading to a collision, every node needs to wait for the medium to become free and then invoke the back off mechanism. For this, each node selects a random back off interval, illustrated by the checked box in FIG. 1, within [0, CW], where CW is called the contention window and is initialized to a value CWmin. The node decrements the backoff timer every idle time slot until the counter reaches 0 and the node sends the packet. The CWmin is doubled on each collision until it reaches a maximum threshold called CWmax.
Beam Forming
Beam forming is a general set of techniques to control the radiation pattern of a radio signal. One way of achieving this is to use several fixed antenna elements. The total antenna pattern can be controlled by adjusting the transmit weights of the signal components radiating from each individual antenna element. The beam forming coefficients can be calculated to direct the transmitted energy towards the position of the intended receiver, while simultaneously reducing the amount of energy radiated in unwanted directions.
Transmit beam forming is a key enabler for enhancing the capacity and the energy efficiency in a cellular network and is therefore of major importance in future radio access systems. The received signal strength is increased due to the increased antenna gain resulting from the beam forming operation. At the same time interference is spread over a smaller area, typically resulting in reduced interference levels for all users in the system. Increased Signal to Interference and Noise, SINR, results in higher bit-rates and higher capacity. Higher SINR in a packet oriented system results in shorter packet transmission times. This also helps to reduce the energy consumption in the system since transmitters and receivers can be put into idle mode during a larger ratio of time.
In the simplest form an antenna radiation pattern can be described as pointing in a certain direction with a certain beam width. The direction of the maximum gain of the antenna pattern (usually denoted boresight) can be described as a vector with a vertical component (usually denoted elevation or antenna tilt) and a horizontal component (usually denoted azimuth). The beam width also has two dimensions, one vertical and one horizontal.
Receive beam forming uses the reciprocity of transmit and receive paths to apply directionality towards the receiver. Like transmit beam forming, one way to achieve directivity is to use a number of fixed antenna elements which phases are controlled to steer the direction of the resultant antenna pattern.
The gain of a directive antenna (i.e. the gain by how much the desired signal is amplified over the signal of an omnidirectional antenna) increases with decreasing beam width. The narrower the generated beam the higher the antenna gain.
A well-known problem of contention based MAC protocols when used together with beam forming are hidden nodes. See FIGS. 2a and 2b for a graphical illustration. In FIG. 2a two transmitters, 20a and 20b, are both contending for the medium—and thus listen to the medium—may not hear each other due to the directive transmissions of the other. At the destination node, 10a,—since both nodes want to communicate with the same node they direct their respective beams towards the common receiver—a collision occurs.
One well known possible way to mitigate this problem is that each transmitter sends prior to the directive transmission an omni-directional pilot signal as illustrated in FIG. 2b. For example, the RTS and CTS described above may be implemented as omnidirectional pilots. Contending transmitter in the neighbourhood can overhear the omni-directional pilot transmission and refrain from accessing the medium.
One drawback with this solution is that it may be overly pessimistic: It avoids all simultaneous transmissions in a neighbour using the same resources. If all transmissions are intended for the same reception node this is also desirable. And all transmissions in the neighbourhood are avoided until the entire message exchange sequence is finished (as described above in the description of the NAV). However, if not all transmissions are intended for the same receiving node this approach becomes overly pessimistic since even non-colliding transmissions are avoided, see FIG. 3. In FIG. 3 two user equipments 20a, 20b want to communicate with two access nodes 10a, 10b, respectively. Since directed into different directions their transmissions do not collide. However, the omni-directional pilot signals sent by the user equipments 20a, 20b are overheard by the user equipments 20b, 20a, respectively, and therefore both user equipments apply a random back-off according to the MAC protocol.